The present invention relates particularly, although by no means exclusively, to a method of starting a molten bath-based smelting process for producing molten metal from a metalliferous feed material in a smelting vessel that has a strong bath/slag fountain generated by gas evolution in the molten bath, with the gas evolution being at least partly the result of devolatilisation of carbonaceous material in the molten bath.
In particular, although by no means exclusively, the present invention relates to a method of starting a process for smelting an iron-containing material, such as an iron ore, and producing iron.
The present invention relates particularly, although by no means exclusively, to a method of starting a smelting process in a smelting vessel that includes a main chamber for smelting metalliferous material.
A known molten bath-based smelting process, generally referred to as the HIsmelt process, is described in a considerable number of patents and patent applications in the name of the applicant.
Another molten bath-based smelting process, referred to hereinafter as the “HIsarna” process, is described in International application PCT/AU99/00884 (WO 00/022176) in the name of the applicant.
The HIsmelt process and the HIsarna process are associated particularly with producing molten iron from iron ore or another iron-containing material.
The HIsarna process is carried out in a smelting apparatus that includes (a) a smelting vessel that includes a main smelting chamber and lances for injecting solid feed materials and oxygen-containing gas into the main chamber and is adapted to contain a bath of molten metal and slag and (b) a smelt cyclone for pre-treating a metalliferous feed material that is positioned above and communicates directly with the smelting vessel.
The term “smelt cyclone” is understood herein to mean a vessel that typically defines a vertical cylindrical chamber and is constructed so that feed materials supplied to the chamber move in a path around a vertical central axis of the chamber and can withstand high operating temperatures sufficient to at least partially melt metalliferous feed materials.
In one form of the HIsarna process, carbonaceous feed material (typically coal) and optionally flux (typically calcined limestone) are injected into a molten bath in the main chamber of the smelting vessel. The carbonaceous material is provided as a source of a reductant and a source of energy. Metalliferous feed material, such as iron ore, optionally blended with flux, is injected into and heated and partially melted and partially reduced in the smelt cyclone. This molten, partly reduced metalliferous material flows downwardly from the smelt cyclone into the molten bath in the smelting vessel and is smelted to molten metal in the bath. Hot reaction gases (typically CO, CO2, H2, and H2O) produced in the molten bath is partially combusted by oxygen-containing gas (typically technical-grade oxygen) in an upper part of the main chamber. Heat generated by the post-combustion is transferred to molten droplets in the upper section that fall back into the molten bath to maintain the temperature of the bath. The hot, partially-combusted reaction gases flow upwardly from the main chamber and enter the bottom of the smelt cyclone. Oxygen-containing gas (typically technical-grade oxygen) is injected into the smelt cyclone via tuyeres that are arranged in such a way as to generate a cyclonic swirl pattern in a horizontal plane, i.e. about a vertical central axis of the chamber of the smelt cyclone. This injection of oxygen-containing gas leads to further combustion of smelting vessel gases, resulting in very hot (cyclonic) flames. Finely divided incoming metalliferous feed material is injected pneumatically into these flames via tuyeres in the smelt cyclone, resulting in rapid heating and partial melting accompanied by partial reduction (roughly 10-20% reduction). The reduction is due to both thermal decomposition of hematite and the reducing action of CO/H2 in the reaction gases from the main chamber. The hot, partially melted metalliferous feed material is thrown outwards onto the walls of the smelt cyclone by cyclonic swirl action and, as described above, flows downwardly into the smelting vessel below for smelting in the main chamber of that vessel.
The net effect of the above-described form of the HIsarna process is a two-step countercurrent process. Metalliferous feed material is heated and partially reduced by outgoing reaction gases form the smelting vessel (with oxygen-containing gas addition) and flows downwardly into the smelting vessel and is smelted to molten iron in the smelting vessel. In a general sense, this countercurrent arrangement increases productivity and energy efficiency.
The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.
The applicant has proposed that the HIsarna process and an oxygen-blown version of the HIsmelt process be started up in a smelting vessel by feeding hot metal (from an external source) into the main chamber of the vessel via the forehearth of the vessel, commencing supplying oxygen-containing gas (typically technical grade oxygen) and solid carbonaceous material (typically coal) and generating heat in the main chamber. This hot start-up method generates heat via spontaneous ignition of combustible material in the main chamber. The applicant has proposed that this initial step in the hot start-up method be followed by the addition of slag-forming agents and, later on, by the addition of metalliferous feed material (such as ferruginous material such as iron ore) into the main chamber.
In pilot plant trials of the HIsarna process that were based on cold technical-grade oxygen as the oxygen-containing gas, coal as the solid carbonaceous material, and iron ore fines as the metalliferous material, the applicant found that such a start-up can fail under certain conditions. By inadvertently allowing a long period of time to pass between charging hot metal and admitting oxygen/coal into the main chamber of the smelting vessel, it was found that coal-oxygen ignition could fail despite that fact that fresh hot metal had recently been poured into the main chamber. This led to an un-combusted mixture of coal and oxygen leaving the smelting vessel, and this in turn triggered a coal dust explosion in a downstream waste heat boiler.
The applicant believes that this type of situation must be avoided since it can lead to serious injury and/or equipment damage. As a consequence of this failed start-up, the applicant subsequently installed a camera in the smelting vessel to observe directly what was causing ignition failure.
Video footage showed that, when hot metal is poured into the main chamber of the smelting vessel, there are spontaneous sparks and small splashes of hot metal which are easily capable of igniting a cold oxygen-coal mixture in the main chamber. However, as time passes, a thin layer of slag builds on the hot metal surface, and hot metal splashing activity gradually dies down. Eventually, the metal becomes completely blanketed with a slag crust, and metal splashing activity stops. If oxygen and coal are fed under these conditions, it is believed that ignition can fail.
The slag is believed to come from two sources: (1) slag left behind in the main chamber of the smelting vessel from previous operations, such as previous smelting campaigns, and (2) oxidation of certain metal species (particularly silicon) in hot metal. The degree to which the latter occurs is a function of how much silicon is present in the charge metal and, in cases where silicon is deliberately increased as part of start-up, this effect is intensified. The important practical conclusion is that a slag layer can always form, and a safe start-up method must accommodate this possibility.
Slag layer formation is a function of vessel geometry, charge metal temperature/composition and vessel condition (e.g. thickness of existing freeze layers on side walls of vessels). When hot metal is poured into a main chamber of a smelting vessel, there is an immediate loss of heat by radiation from the relatively quiescent bath surface to the side walls of the main chamber that are above the hot metal. These side walls may be refractory walls. In the case of the smelting vessel of particular interest to the applicant, the side walls include water-cooled panels. Metal, by virtue of having a high density and a relatively low viscosity under these conditions, tends to circulate within itself. This suppresses any initial tendency to form a solidified or highly viscous uniform crust across its top surface. Slag, on the other hand, tends to float as a more or less uniform thin layer on top of the metal. As it loses heat by radiation, its viscosity rises and it becomes sticky. Under these conditions an insulating slag crust (in effect an insulating “blanket”) is effectively formed on top of the hot metal. This is considered by the applicant to be the key mechanism associated with the ability of slag to compromise oxygen-coal ignition under start-up conditions. This is a time-related mechanism.
Understanding the time-scale associated with the formation of this slag crust is critical for safe plant operation. For the pilot plant described herein, the (metal) bath diameter was around 2.6 m and the top space was defined by fully water-cooled panels in the side walls and the roof of the smelting vessel. A provisional (sacrificial) cast/gunned refractory layer was present on the water panels at the time. In the trial involving the failed start-up (leading to the coal dust explosion), metal was charged into the main chamber of the vessel and 7 separate attempts were made to start the process by adding oxygen and coal to the main chamber. Of these, 6 were made within the first 2 hours after charging, and each time it was possible to show that ignition had indeed taken place (from water panel heat load and gas composition data) but the start-up attempt had subsequently failed for reasons unrelated to ignition. The 7th (and last) attempt was made around 2.5 hours after completion of the hot metal charge. It is this attempt that led to final ignition failure and the resulting coal dust explosion.
For this particular smelting facility, there appears to be a “safe” ignition time-window of around 1-2 hours after completion of hot metal charging (during which spontaneous ignition of oxygen and coal can be reasonably assured). Beyond this, safe ignition is not assured and an alternate cold start-up method needs to be followed. The cold start-up method is described in a companion International application entitled “Starting a Smelting Process” lodged in the name of the applicant on the same day as the International application for the present invention.
Translation of this specific time-window to other smelting facilities must be undertaken with care, giving due consideration to the factors discussed above (vessel geometry, charge metal conditions etc).